1、Designation: C1368 10Standard Test Method forDetermination of Slow Crack Growth Parameters ofAdvanced Ceramics by Constant Stress-Rate StrengthTesting at Ambient Temperature1This standard is issued under the fixed designation C1368; the number immediately following the designation indicates the year
2、 oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method covers the determination of slow crack
3、growth (SCG) parameters of advanced ceramics by usingconstant stress-rate rectangular beam flexural testing, or ring-on-ring biaxial disk flexural testing, or direct tensile strength,in which strength is determined as a function of applied stressrate in a given environment at ambient temperature. Th
4、estrength degradation exhibited with decreasing applied stressrate in a specified environment is the basis of this test methodwhich enables the evaluation of slow crack growth parametersof a material.NOTE 1This test method is frequently referred to as “dynamicfatigue” testing (Refs (1-3)2) in which
5、the term “fatigue” is usedinterchangeably with the term “slow crack growth.” To avoid possibleconfusion with the “fatigue” phenomenon of a material which occursexclusively under cyclic loading, as defined in Terminology E1823, thistest method uses the term “constant stress-rate testing” rather than“
6、dynamic fatigue” testing.NOTE 2In glass and ceramics technology, static tests of considerableduration are called “static fatigue” tests, a type of test designated asstress-rupture (See Terminology E1823).1.2 Values expressed in this test method are in accordancewith the International System of Units
7、 (SI) and IEEE/ASTM SI10.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations pri
8、or to use.2. Referenced Documents2.1 ASTM Standards:3C1145 Terminology of Advanced CeramicsC1161 Test Method for Flexural Strength of AdvancedCeramics at Ambient TemperatureC1239 Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distribution Parameters for AdvancedCeramicsC1273 Tes
9、t Method for Tensile Strength of MonolithicAdvanced Ceramics at Ambient TemperaturesC1322 Practice for Fractography and Characterization ofFracture Origins in Advanced CeramicsC1499 Test Method for Monotonic Equibiaxial FlexuralStrength of Advanced Ceramics at Ambient TemperatureE4 Practices for For
10、ce Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical TestingE337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)E1823 Terminology Relating to Fatigue and Fracture Test-ingIEEE/ASTM SI 10 American National Sta
11、ndard for Use ofthe International System of Units (SI): The Modern MetricSystem3. Terminology3.1 DefinitionsThe terms described in TerminologiesC1145, E6, and E1823 are applicable to this test method.Specific terms relevant to this test method are as follows:3.1.1 advanced ceramic, na highly enginee
12、red, high-performance, predominately nonmetallic, inorganic, ceramicmaterial having specific functional attributes. (C1145)1This test method is under the jurisdiction of ASTM Committee C28 onAdvanced Ceramics and is the direct responsibility of Subcommittee C28.01 onMechanical Properties and Perform
13、ance.Current edition approved Dec. 1, 2010. Published January 2011. Originallyapproved in 1997. Last previous edition approved in 2006 as C1368 06. DOI:10.1520/C1368-10.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For referenced ASTM standards, vis
14、it the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International,
15、 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.2 constant stress rate,s, na constant rate of maximumstress applied to a specified beam by using either a constantloading or constant displacement rate of a testing machine.3.1.3 environment, nthe aggregate of c
16、hemical speciesand energy that surrounds a test specimen. (E1823)3.1.4 environmental chamber, nthe container of bulkvolume surrounding a test specimen. (E1823)3.1.5 equibiaxial flexural strength F/L2, nthe maximumstress that a material is capable of sustaining when subjected toflexure between two co
17、ncentric rings.3.1.5.1 DiscussionThis mode of flexure is a cupping ofthe circular plate caused by loading at the inner load ring andouter support ring. The equibiaxial flexural strength is calcu-lated from the maximum-load of a biaxial test carried torupture, the original dimensions of the test spec
18、imen, andPoissons ratio. (C1499)3.1.6 flexural strength, sf, na measure of the strength of aspecified beam specimen in bending determined at a givenstress rate in a particular environment.3.1.7 fracture toughness, na generic term for measures ofresistance to extension of a crack. (E1823)3.1.8 inert
19、strength, na measure of the strength of aspecified strength test specimen as determined in an appropri-ate inert condition whereby no slow crack growth occurs.3.1.8.1 DiscussionAn inert condition may be obtained byusing vacuum, low temperatures, very fast test rates, or anyinert mediums.3.1.9 slow c
20、rack growth (SCG), nsubcritical crack growth(extension) which may result from, but is not restricted to, suchmechanisms as environmentally-assisted stress corrosion ordiffusive crack growth.3.1.10 strength-stress rate curve, na curve fitted to thevalues of strength at each of several stress rates, b
21、ased on therelationship between strength and stress rate: log sf= 1/(n +1)log s + log D. (See Appendix X1.)3.1.10.1 DiscussionIn the ceramics literature, this is oftencalled a dynamic fatigue curve.3.1.11 strength-stress rate diagram, na plot of strengthagainst stress rate. Both strength and stress
22、rate are plotted onlog-log scales.3.1.12 stress intensity factor, KI, nthe magnitude of theideal-crack-tip stress field (stress-field singularity) subjected tomode I loading in a homogeneous, linear elastic body.(E1823)3.1.13 tensile strength F/L2, nSuthe maximum tensilestress which a material is ca
23、pable of sustaining.3.1.13.1 DiscussionTensile strength is calculated from themaximum force during a tension test carried to rupture and theoriginal cross-sectional area of the specimen. (C1273)3.2 Definition of Term Specific to This Standard:3.2.1 slow crack growth parameters, n and D, ntheparamete
24、rs estimated as constants in the flexural strength-stressrate equation, which represent the degree of slow crack growthsusceptibility of a material. (See Appendix X1.)4. Significance and Use4.1 For many structural ceramic components in service,their use is often limited by lifetimes that are control
25、led by aprocess of SCG. This test method provides the empiricalparameters for appraising the relative SCG susceptibility ofceramic materials under specified environments. Furthermore,this test method may establish the influences of processingvariables and composition on SCG as well as on strengthbeh
26、avior of newly developed or existing materials, thus allow-ing tailoring and optimizing material processing for furthermodification. In summary this test method may be used formaterial development, quality control, characterization, andlimited design data generation purposes. The conventionalanalysi
27、s of constant stress-rate testing is based on a number ofcritical assumptions, the most important of which are listed inthe next paragraphs.4.2 The flexural stress computation for the rectangular beamtest specimens or the equibiaxial disk flexure test specimens isbased on simple beam theory, with th
28、e assumptions that thematerial is isotropic and homogeneous, the moduli of elasticityin tension and compression are identical, and the material islinearly elastic. The average grain size should be no greaterthan one fiftieth of the beam thickness.4.3 The test specimen sizes and fixtures for rectangu
29、larbeam test specimens should be in accordance with Test MethodC1161, which provides a balance between practical configura-tions and resulting errors, as discussed in Refs (4, 5). Onlyfour-point test configuration is allowed in this test method forrectangular beam specimens. Three-point test configu
30、rationsare not permitted. The test specimen sizes and fixtures for disktest specimens tested in ring-on-ring flexure should be chosenin accordance with Test Method C1499. The test specimens fordirect tension strength testing should be chosen in accordancewith Test Method C1273.4.4 The SCG parameters
31、 (n and D) are determined by fittingthe measured experimental data to a mathematical relationshipbetween strength and applied stress rate, log sf= 1/(n+1) log s+ log D. The basic underlying assumption on the derivation ofthis relationship is that SCG is governed by an empiricalpower-law crack veloci
32、ty, v=AKI/KICn(see Appendix X1).NOTE 3There are various other forms of crack velocity laws whichare usually more complex or less convenient mathematically, or both, butmay be physically more realistic (Ref (6). It is generally accepted thatactual data cannot reliably distinguish between the various
33、formulations.Therefore, the mathematical analysis in this test method does not coversuch alternative crack velocity formulations.4.5 The mathematical relationship between strength andstress rate was derived based on the assumption that the slowcrack growth parameter is at least n $ 5 (Refs (1, 7, 8)
34、.Therefore, if a material exhibits a very high susceptibility toSCG, that is, n 2000 MPa/s) mayremain unchanged so that a plateau is observed in the plot ofstrength versus stress rate (Ref (7). If the strength datadetermined in this plateau region are included in the analysis,a misleading estimate o
35、f the SCG parameters will be obtained.Therefore, the strength data in the plateau shall be excluded asdata points in estimating the SCG parameters of the material.This test method addresses for this factor by recommendingthat the highest stress rate by #2000 MPa/s.NOTE 5The strength plateau of a mat
36、erial can be checked bymeasuring an inert strength in an appropriate inert medium.NOTE 6When testing in environments with less than 100% concen-tration of the corrosive medium (for example, air), the use of stress ratesgreater than 1 MPa/s can result in significant errors in the slow crackgrowth par
37、ameters due to averaging of the regions of the slow crackgrowth curve (16). Such errors can be avoided by testing in 100%concentration of the corrosive medium (for example, in water instead ofhumid air). For the case of 100% concentration of the corrosive medium,stress rates as large as 2000 MPa/s m
38、ay be acceptable.5.3 Surface preparation of test specimens can introducefabrication flaws which may have pronounced effects on SCGbehavior. Machining damage imposed during specimen prepa-ration can be either a random interfering factor or an inherentpart of the strength characteristics to be measure
39、d. Surfacepreparation can also lead to residual stress. Universal orstandardized test methods of surface preparation do not exist. Itshould be understood that the final machining steps may ormay not negate machining damage introduced during the earlycoarse or intermediate machining steps. In some ca
40、ses, speci-mens need to be tested in the as-processed condition tosimulate a specific service condition. Therefore, specimenfabrication history may play an important role in slow crackgrowth as well as in strength behavior.6. Apparatus6.1 Testing MachineTesting machines used for this testmethod shal
41、l conform to the requirements of Practices E4.Specimens may be loaded in any suitable testing machineprovided that uniform test rates, either using load-controlled ordisplacement-controlled mode, can be maintained. The loadsused in determining strength shall be accurate within 61.0 %at any load with
42、in the selected load rate and load range of thetesting machine as defined in Practices E4. The testing machineshall have a minimum capability of applying at least four testrates with at least three orders of magnitude, ranging from 101to 102N/s for load-controlled mode and from 107to 104m/sfor displ
43、acement-controlled mode.6.2 Test Fixtures, Four-Point Rectangular Beam FlexureThe configurations and mechanical properties of test fixturesshould be in accordance with Test Method C1161. The mate-rials from which the test fixtures including bearing cylindersare fabricated shall be effectively inert
44、to the test environmentso that they do not react with or contaminate the environment.NOTE 7For testing in water, for example, it is recommended that thetest fixture be fabricated from stainless steel which is effectively inert towater. The bearing cylinders may be machined from hardenable stainlesss
45、teel (for example, 440C grade) or a ceramic material such as siliconnitride, silicon carbide, or alumina.C1368 1036.2.1 Four-Point FlexureThe four-point-14 point fixtureconfiguration as described in 6.2 of Test Method C1161 shallbe used in this test method. Three-point flexure is not permit-ted. The
46、 test fixtures shall be stiffer than the specimen, so thatmost of the crosshead or actuator travel is imposed onto thespecimen.6.3 Test Fixtures, Equibiaxial Disk Flexural StrengthTheconfigurations and mechanical properties of test fixtures shouldbe in accordance with Test Method C1499. The material
47、s fromwhich the test fixtures including bearing cylinders are fabri-cated shall be effectively inert to the test environment so thatthey do not react with or contaminate the environment. SeeNote 7. The test fixtures shall be stiffer than the specimen, sothat most of the crosshead or actuator travel
48、is imposed ontothe specimen.6.4 Test Fixtures, Tensile StrengthThe configurations andmechanical properties of test fixtures should be in accordancewith Test Method C1273. The materials from which the testfixtures including bearing cylinders are fabricated shall beeffectively inert to the test enviro
49、nment so that they do notreact with or contaminate the environment. See Note 7. Thetest fixtures shall be stiffer than the specimen, so that most ofthe crosshead or actuator travel is imposed onto the specimen.6.5 Data AcquisitionAccurate determination of both frac-ture load and test time is important since it affects not onlyfracture strength but applied stress rate. At the minimum, anautographic record of applied load versus time should bedetermined during testing. Either analog chart recorders ordigital data acquisition systems c
